High voltage current transformer



July 15, 1969 v. N. STEWART 3,456,220

HIGH VOLTAGE CURRENT TRANSFORMER Filed April 25, 1968 2 Sheets-Sheet 1 INVENTOR. VINCENT /V. JTEWAR 7 BY 7 mm ATTORNEY July 15, 1969 v. N. STEWART HIGH VOLTAGE CURRENT TRANSFORMER 2 Sheets-Sheet 2 Filed April 25, 1968 mvz/vron VINCENT N. 5 TE wmr, BY M A TTORNE Y United States Patent 3,456,220 HIGH VOLTAGE CURRENT TRANSFORMER Vincent N. Stewart, Springfield, Pa, assignor to General Electric Company, a corporation of New York Filed Apr. 25, 1968, Ser. No. 724,126 Int. Cl. Htllf 27/10, 27/08, /04

U.S. Cl. 33658 9 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a high voltage current transformer and, more particularly, a current transformer of this type in which means is provided for cooling the primary winding of the transformer by causing a coolant to flow therethrough.

The particular type of current transformer that I am concerned with comprises a tank and a tubular insulator mounted atop the tank. The primary winding of the transformer comprises the series combination of a first tubular conductive arm extending through the tubular insulator into the tank, a loop-shaped conductor within the tank, and an inner conductive arm mounted within said tubular arm in substantially coaxial relationship therewith. It is highly desirable that the cross-section of the coaxiallydisposed conducting arms be kept as small as possible since this enables the expensive tubular insulator surrounding them to be kept small. But the smaller the crosssection of the conductive arms, the more heating that occurs to the higher electrical resistance. However, by providing suitable cooling means, I can reduce the crosssection to the desired extent without causing excessive heating.

For effecting the desired cooling, I make the inner conductive arm of a tubular configuration and cause coolant to pass vertically upward through the space within this arm. For this cooling scheme to be effective, the inlet to the space should be located at the lowermest end of the conductive arm so that effective flow takes place in this region at the lowermost end. But locating the inlet in this region can cause dielectric strength problems because this is a region of high electric stress Where the presence of irregular surfaces typically associated with such an inlet could lead to an electrical breakdown.

An object of my invention is to overcome the dielectric strength problem associated with providing an inlet for coolant in this region of high electric stress.

In a preferred form of my invention, the tank and tubular insulator are filled with a pressurized insulating gas. A terminal portion of one of the arms of the primary winding projects through a plate mounted on the upper end of the tubular insulator. In one prior construction of this type, a sliding seal has been provided between this terminal portion and the plate to permit thermallyinduced movement of the arm without allowing leakage of the pressurized gas along the arm. In another prior construction, a sealed termination space has been provided above the plate, and external connections have been made through a flexible connection in the sealed space.

Another object of my invention is to eliminate the need for such a sliding seal or extra termination space and flexible connection.

To permit assembly of the cores of the current transformer about the primary conductor, it has been customary to provide a pair of separable joints in the primary conductor. After the cores are in place and these joints in the primary are assembled, it has heretofore been customary to smooth the external surfaces of the joints by some type of abrading operation in order to avoid dielectric impairment due to surface irregularities. This abrading operation is awkward to perform and can be expensive and time-consuming.

Another object of my invention is to construct these joints in such a way that they do not require such an abrading operation after assembly.

In carrying out my invention in one form, I provide a pair of joining structures for electrically and mechanically interconnecting the lower ends of the tubular conductive arms and the opposite ends of the loop-shaped primary conductor, respectively. An opening is provided through the joining structure that is connected to the inner tubular arm for defining an inlet to the space within said tubular arm at its lower end. An electrostatic shield of smoothlycurved external configuration covering the sharp edges in the region of said inlet is provided to reduce the electric stresses thereadjacent. The shield is spaced from the joining structure to define an entry passage therebetween through which fluid is led to said inlet. The entrance to said entry passage is located in a region of relatively low electric stress compared to that present at the exterior of said shield immediately opposite said inlet.

In a preferred form of the invention, means is provided for fixedly securing the upper ends of said arms to the upper ends of said tubular insulator. The lower ends of the arms are free to move with respect to each other in a direction longitudinal of the arms in response to different thermally-induced expansions of the two arms. The electrostatic shield is so shaped that no sharp corners exposed to said high electric field appear despite thermally-induced relative movement of the lower ends of said arms.

For a better understanding of the invention, reference may be had to the following description taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a side elevational view partly in section of a current transformer embodying one form of my invention.

FIG. 2 is an enlarged sectional view of a portion of the primary winding of the current transformer of FIG. 1.

FIG. 3 is a sectional view along the line 33 of FIG. 2.

FIG. 4 is a sectional view on a reduced scale taken along the line 44 of FIG. 2.

FIG. 5 is a schematic view on a reduced scale taken along the line 5-5 of FIG. 1.

Referring now to FIG. 1, the high voltage current transformer shown therein comprises a grounded metal tank 12 comprising a metal top plate 14 and a cupshaped body portion 15, across the mouth of which the top plate extends. The body portion is detachably connected to the top plate by suitable means, such as screws 16, extending through openings in the top plate and threaded into a flange 18 on the body portion.

Mounted on the top plate 14 is a vertically extending tubular insulating column 20, preferably of porcelain.

This tubular column 20 has an annular flange 21 suitably bonded to its lower end and bolted to the top plate 14. An opening 23 in top plate 14 is disposed in alignment with the bore of tubular insulating column 20.

The primary winding of the current transformer comprises two spaced-apart conductive arms 30 and 31 extending through tubular insulator 20* into tank 12. Each arm 30 and 31 is an elongated tubular member and is coaxially disposed with respect to the other arm. The primary winding further comprises a loop-shaped conductor 32 located within the tank and electrically interconnecting the lower ends of conductive arms 30 and 31.

The loop-shaped conductor 32 is a generally tubular member comprising two upper portions 32a and 32b, each of which is detachably connected to a lower portion 32c of generally semi-toroidal form. The coupling between the lower portion 320 and the upper portion 32a comprises a pair of plugs 34 and 35 respectively fitted into the hollow ends of these two portions and suitably welded in place. A screw 37, located inside the tubular conductor 32, extends through the lower plug 35 into a threaded hole in the upper plug 34 to clamp together the plugs 34, 35 and hence the portions 32a and 320 of the loop-shaped conductor 32. An identical joint having correspondingly-designated parts is used to clamp together portions 32b and 320 of the loop-shaped conductor 32. Access to the screws 37 is had by means of opening 38 provided in the tubular loop-shaped conductor 32 in a region of relatively low electric stress.

For joining the opposite ends of the loop-shaped conductor 32 to the conductive arms 31 and 31, respectively, I provide a pair of connecting means comprising inserts 40 and 42. These inserts are occasionally referred to hereinafter as joining members. Referring to FIG. 2, insert 40 has a plug portion at its right-hand end fitted into the tubular portion 32a of the primary conductor. This plug portion is suitably welded to conductor 32a at 43, and the external surface of the assembly in this region is ground to make it smooth and free of surface irregularities that could cause dielectric strength problems. At its left-hand end, insert 40 has an integral bar portion 41 containing a circular opening that receives the lower end of inner tubular arm 31. Along its lowermost face, the inner tubular arm 31 is suitably welded to the ban portion 41 of insert 40.

Referring to FIG. 2, the other insert 42 has a plug portion at its left-hand end fitted into the tubular portion 32b of the primary conductor and welded in place 1n this position. At its right-hand end, insert 42 has an integral bar portion 43 positioned above the bar port on 41 of the insert 40 and spaced therefrom. Bar portion 43 has a large opening 44 therein through which inner tubular conductor 31 extends in spaced-apart relationship thereto. At its lowermost face, the outer tubular conductor 30 abuts against the bar portion of plug 42 and 1s suitably welded thereto. A sleeve 46 of suitable insulat ng material surrounds the inner conductor 31 in a position within opening 44, and has a flange 47 positioned between the two bar portions 41 and 43. This insulating sleeve serves to prevent current from flowing directly between the inner and outer tubular conductors 31 and 30, thus forcing all current passing between conductors 31 and 30 to follow a path around the loop-shaped conductor 32.

In the illustrated embodiment (FIG. 1), two secondary windings 140 and 142 are shown within tank 12. A different number of secondary windings, if required, could be accommodated. Each of the secondary windings is of a conventional design and, as such, it is wound about its own magnetic core 144, which encompasses the loopshaped primary conductor 32 in spaced-apart relationship thereto. The turns of each of the secondary windings are electrically insulated from each other and from the core in a conventional manner. The cores 144' are at ground potential, as is one point on each of the secondary windings. Conventional secondary leads (not shown) extend from the secondary windings through the wall of tank 12 via a sealed terminal plate (not shown).

The cores and the secondary windings are supported by a U-shaped cradle 90 schematically shown in FIG. 5. This cradle 90, which is not a part of my invention, is

described in greater detail and claimed in co-pending application Ser. No. 724,125Berg, filed Apr. 25, 1968, and assigned to the assignee of the present invention. In general, this cradle comprises a pair of spaced-apart legs 92 detachably secured to top plate 14 and a generally semi-cylindrical saddle portion 94 joined at its respective opposite ends to the legs. The cores 144 and secondary windings 140, 142 are mounted on the saddle portion 94 and are detachably secured thereto by suitable fastening means.

The secondary structure is enclosed by a grounded metal shield that serves to electros-tatically shield the secondary structure. In this regard, the shield 100 is of a generally toroidal form with a smoothly curved exterior surface to reduce electric stress concentrations theread jacent. Shield 1110 is preferably made of two halves suitably held together, with electrical insulation between them to prevent the shield from forming a short circuited turn around cores 44.

Because the inner periphery of the high voltage loopshaped primary conductor 32 is located closely adjacent the grounded shielding means 100, it will be apparent that the inner periphery of the loop-shaped conductor is in a region of high electric stress, as will be referred to hereinafter.

The tank 12 and the insulating column 20 are filled with a suitable gaseous insulating material for insulating the high voltage primary winding from the grounded portions of the current transformer assembly. This gaseous insulating material is preferably sulphur hexafluoride at a pressure of several atmospheres. Pressure tight joints throughout the assembly prevent any significant leakage of the pressurized gas therefrom.

For cooling the primary winding, I cause a continuous flow of this insulating gas to pass upwardly through the two tubular arms 31 and 30. For inducing this flow, openings are provided in each of the tubular conductors 30 and 31 at both their top and bottom ends. Referring to FIG. 2, the lower openings in the outer conductor 30 are shown at 50 and the upper openings at 51. In the inner conductor 31, the lower opening 52 is at the lowermost end of the tubular conductor, and the upper openings are at 53. Primary current flowing through tubular conductors 31 and 30 heats the gas within the tubular conductors. The heated gas rises within the tubular conductors, exits from the tubular conductors through top openings 51 and 53, and is replaced by relatively cool gas entering the tubular conductors 30 and 31 through the bottom openings 50 and 52. The net result is a continuous flow of cooling gas upwardly through the tubular conductors 30 and 31, as is indicated by arrows 56. This flow of cooling gas is highly desirable because it enables the conductors to carry relative large amounts of current per unit of cross-section without overheating. Thus, these conductors can be made relatively small in cross section, and the surrounding tubular insulator 20 can be made correspondingly small in diameter, which is a considerably economic advantage.

The heated gas leaving the tubular conductors through top openings 51 and 53 cools upon entering the large enclosure space external to the conductors and moves downwardly as it cools, being replaced by additional heated gas leaving through openings 51 and 53. Thus, there is a continuous flow of cooling gas upwardly through the interior of the tubular conductors and downwardly within column 20 about the exterior of the outer conductor 30.

For cooling to be effective for the entire length of the inner arm 31, it is important that the inlet opening for coolant admitted to the inner arm be located adjacent its lowermost end, as is the case with my inlet opening 52. But locating the inlet in this region can cause dielectric strength problems because this is ordinarily a region of high electric stress, as was noted hereinabove, where the presence of irregular surfaces typically associated with such an inlet could lead to an electric breakdown.

For overcoming this dielectric strength problem, I provide in this region a thin-walled electrostatic shield 60 having a smoothly-curved external configuration. This electrostatic shield is suitably welded to inert 40 but is spaced from inlet opening 52. The shield 60 has a gen erally U-shaped cross section as viewed in the horizontal plane of FIG. 4. The shield covers the sharp edges adjacent the inlet opening and eliminates the electric field in the region immediately adjacent these edges. The space 63 between the shield 60 and the bar portion 41 of insert 40 serves as an entry passage between these parts through which fluid is led to the inlet 52. The entrance 65 to this entry passage is along the upper edge of shield 60. More specifically, the upper edge of shield 60 is spaced from the flange 47 of the insulating sleeve 46, and the resulting space along this entire upper edge forms an entrance through which cooling gas flows into the passage 63 and inlet 52. It is to be noted that the entrance 65 is located at the sides of the loop-shaped conductor 62 where the electric stress is relatively low, rather than at the inner periphery of the loop-shaped conductor, where much higher electric stresses are present due to the proximity of grounded shield 100, as pointed out hereiriabove. Thus, even though there are edges around the shield defining the entrance 65, these edges are out of the high electric stress and do not create dielectric strength problems.

A similar electrostatic shield 70 of smoothly-curved external configuration is welded to the other insert 42 to cover the sharp edges of this insert in the region of insert 40. The welds between the shields and inserts are suitably ground to provide a smooth external surface free of sharp projections in this region. This grinding operation is performed prior to assembly of the primary into the current transformer.

It is to be noted that the insert 40 and its electrostatic shield 60 are not attached to the other insert 42 or the other shield 70 and are free to move downwardly through a limited amount of travel with respect to said other insert 42 and shield 70. In the past, it has been customary to secure the inserts together and prevent such relative movement in order to prevent any protuberances from developing as a result of such movement in the high stress region at the inner periphery of the loop-shaped conductor. But with the smoothly curved shields 60 and 70 present, no significant protuberances are developed as a result of such movement. Even if shield 60 moves slightly out of alignment with shield 70, the smooth curve on the shields prevents excessive concentration of electric field in this region.

Because such relative movement can be tolerated in the region at the lower ends of conductive arms 30 and 31, it becomes feasible to fix the upper ends of both the arms 30 and 31. In this regard, note that the inner conductor will tend to run at a higher temperature than the outer conductor and will tend to expand longitudinally to a greater extent in response to a given rise in current. With the upper ends of the conductors 30, 31 fixed, as will soon be described, this greater expansion of inner conductor 31 will cause its lower end to move downwardly with respect to the lower end of the outer conductor 30. But this will not cause objectionable protuberances to develop because the electrostatic shield 60 is present to maintain effectively smooth surface conditions in this region despite such downward shifting.

For fixing the upper ends of conductive arms 30 and 31, I provide a metal plate 75 which extends across the top of tubular insulator 20 and is bolted to a flange on the insulator. The outer tubular conductor 30 has a flange 76 that is attached to this plate 75 by suitable screws 76a. The inner conductor 31 extends through plate 75 in spaced relationship thereto. An insulating sleeve 77 with a flange 78 thereon surrounds conductor 31 where it passes through plate 75. A shoulder 80 on inner conductor 31 bears against the flange 78 of the insulating sleeve 77 at its bottom side. A nut 82 threaded on the upper end of conductor 31 bears against washers 83 and an insulating ring 84 that is seated on the plate 75. When nut 82 is tightened, flange78, plate 75, and insulating ring 84 are clamped between the shoulder and nut 82, thereby fixing the inner conductor 31 to plate 75 and electrically insulating it therefrom. A suitable seal 85 is provided between washer 83 and conductor 31 to prevent leakage along the conductor. No significant sliding motion of the conductor 31 with respect to washer 83 takes place at this seal 85 since the parts are all clamped together in this region.

A "significant advantage derived from fixing the upper ends of the conductive arms 30 and 31 is that I am able to eliminate the need for a sliding seal in this region and the need of additional flexible connectors. Heretofore, one of the arms has usually been longitudinally movable with respect to-the end plate, and it was necessary to provide a sliding seal in this region to permit such movement without allowing leakage of the pressurized insulating gas. But since the arms 30 and 31 of my current transformer are fixed with respect to plate 75, there is no longer any need for such a sliding seal.

The joints 34, 35, 37 between the upper and lower portions of the loop-shaped primary conductor are used to facilitate assembly of the secondary about the primary. In this connection, assume that the cup-shaped body portion 15 of tank 12 is not present and that the primary winding without the bottom portion 320 of the loopshaped conductor is present in its position shown. The next step in the assembly operation is to incorporate the secondary structures 140, 142, 144. This is done by mounting the secondary structure in cradle and then attaching the cradle to the top plate 14. Then the bottom portion 32c of the primary winding is inserted through the window of the secondary structure and is secured in place by inserting screws 37 in the two joints 34, 35, 37 and tightening them. Access to the screws 37 is had through openings 38 (FIG. 1). Since the screws 37 are located internally of the tubular conductor 32, they are in a zero electric stress region. The exterior surface of the lower portion 320 substantially aligns with the exterior surface of the adjacent upper portion and thus there are no significant external surface irregularities in this region to produce dielectric strength problems. Because the access openings 38 are along the outer periphery of the primary conductor in positions relatively remote from both the tank 15 and the secondary shielding 100, they are in a low electric stress region and do not produce dielectric strength problems. It should be apparent from the above that the joints 34, 35, 37 can be easily assembled without the need for any grinding of their external surfaces after assembly and yet without causing any surface irregularities to be present in high stress regions.

While I have shown and described a particular embodiment of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made Without departing from my invention in its broader aspects; and I therefore, intend in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a high voltage current transformer comprising a tank, a vertically-extending tubular insulator mounted atop the tank; and an insulating fluid within said tank and tubular insulator;

(a) a primary winding comprising the series combination of an outer tubular conductive arm extending through said tubular insulator, a loop-shaped conductor within said tank, and an inner tubular conductive arm mounted within said outer arm in substantially coaxial relationship therewith,

(b) a pair of joining structures for electrically and mechanically interconnecting the lower ends of said arms and the opposite ends of said loop-shaped conductor, respectively,

(0) an opening through the one joining structure connected to said inner tubular arm for defining an inlet to the space within said inner arm at the lower end of said inner arm,

(d) an opening near the top of said inner tubular arm defining an exit from the space within said inner tubular arm, whereby heated insulating fluid within said space can leave said space through said exit and be replaced by relatively cool insulating fluid entering said space through said inlet,

(e) an electrostatic shield of smoothly-curved external configuration connected to said primary winding and covering the sharp edges in the region of said inlet to reduce the electric stresses there-adjacent,

(f) means defining an entry passage between said shield and said joining structure through which fluid is led to said inlet,

(g) said entry passage having an entrance located in a region of relatively low electric stress compared to that present at the exterior of said shield immediately opposite said inlet.

2. The assembly of claim 1 in which said shield is a thinwalled member enveloping a portion of said one joining structure and having an edge region spaced therefrom to define said entrance.

3. The current transformer of claim 1 in which:

(a) means is provided for fixedly securing the upper ends of said arms to the upper end of said tubular insulator,

(b) the lower ends of said arms are free to move with respect to each other in a direction longitudinal of the arms in response to different thermally-induced expansions of the two arms, and

(c) said shield is so shaped that no sharp corners exposed to the high electric at the inner periphery of said loop-shaped conductor field are developed despite thermally-induced relative movement of the lower ends of said arms.

4. The current transformer of claim 1 in combination with a second electrostatic shield of smoothly-curved external configuration secured to the other of said joining structures at the inner periphery of said loop-shaped conductor adjacent said first electrostatic shield for shielding the adjacent sharp edges on said other joining structure.

5. In a high voltage current transformer comprising a tank and a vertically-extending tubular insulator mounted atop said tank;

(a) a primary winding comprising the series combination of a tubular conductive arm extending through said tubular insulator, a loop-shaped conductor within said tank, and an elongated conductive arm mounted within said tubular arm in substantially coaxial relationship therewith,

(b) means for tfixedly securing the upper ends of said arms to the upper end of said tubular insulator,

(c) the lower ends of said arms being free to move with respect to each other in a direction longitudinally of the arms in response to different thermallyinduced expansions of the two arms,

((1) a pair of connecting means for electrically and mechanically interconnecting the lower ends of said arms and the opposite ends of said loop-shaped conductor, respectively,

(e) means causing the electric field to be high in the region of said connecting means at the inner periphery of said loop-shaped conductor comprising grounded secondary structure located closely adjacent said region,

(f) said connecting means having external surfaces of smoothly-curved configuration devoid of sharp edges in said region of high electric field, said surfaces being so shaped that no sharp corners are exposed to said high electric field despite thermally-induced relative movement of the lower ends of said arms.

6. The current transformer of claim 5 in which:

(a) one of said connecting means comprises a conductive member having an opening therein for receiving said inner conductive arm,

(19) said inner conductive arm is a tubular member having a longitudinally-extending interior space and an inlet thereto aligned with said opening for allowing coolant to enter said interior space,

(c) and said inner conductive arm has an exit from said interior space adjacent its upper end to provide an exit passage for heated coolant leaving said interior space.

7. The current transformer of claim 5 in which:

(a) said inner conductive arm is a tubular member having a cooling passage extending vertically therethrough,

(b) one of said connecting means comprises a first joining member secured to said inner conductive arm and having an opening therethrough receiving said inner conductive arm,

(c) the other of said connecting means comprises a second joining member having an opening, therethrough through which said inner conductive arm extends in spaced-apart relationship,

(d) and solid insulation interposed between said inner conductive arm and the walls of the opening in said second joining member for preventing current from flowing directly between said arms without traversing said loop-shaped conductor.

8. The current transformer of claim 5 in which:

(a) said loop-shaped conductor comprises a lower portion and two upper portions, the upper portions being respectively connected to said conductive arms,

(b) coupling means is provided for detachably connecting said lower portion to said upper portions at opposite ends of the lower portion, and providing joints of substantially smooth external configuration along the inner periphery of said loop-shaped conductor,

(c) said loop-shaped primary conductor is hollow in the regions adjacent said coupling means,

(d) said coupling means comprises fastening means disposed within the hollow regions of said loopshaped conductor for securing said upper and lower portions together,

(e) access openings in the hollow regions of said loopshaped conductor spaced from said coupling means for aifording access to said fastening means,

(f) said access openings being located in regions of low electric stress compared to that present at the inner periphery of said loop-shaped conductor.

9. In a high voltage current transformer comprising a tank and a vertically-extending tubular insulator mounted atop said tank:

(a) a primary winding comprising the series combination of a tubular conductive arm extending through said tubular insulator, a loop-shaped conductor within said tank, and an elongated conductive arm mounted within said tubular arm in substantially coaxial relationship therewith,

(b) a pair of connecting means for electrically and mechanically interconnecting the lower ends of said arms and the opposite ends of said loop-shaped conductor, respectively,

(c) means causing the electric field to be relatively high at the inner periphery of said loop-shaped conductor comprising grounded secondary structure located closely adjacent said inner periphery,

(d) said loop-shaped conductor comprising a lower portion and two upper portions, the upper portions being respectively connected to said conductive arms,

(e) coupling means for detachably connecting said lower portion to said upper portions at opposite ends of the lower portion and providing joints therebetween of substantially smooth external configura- 9 10 tion along the inner periphery of said loop-shaped References Cited COIIdHCtOI, UNITED STATE PA EN s (f) said loop-shaped primary conductor being hollow S T T in the regions adjacent said coupling means, 31281521 10/1966 Wllson XR (g) said coupling means comprising fastening means 5 312991383 1/1967 Conner at 336-173 XR disposed Within the hollow regions of said loopshaped conductor for securing said upper and lower LEWIS MYERS Primary Exammer portions together, T. I. KOZMA, Assistant Examiner (h) access openings in the hollow regions of said loop- U S C1 X R shaped conductor spaced from said coupling means 10 336 60 84 174 for affording access to said fastening means,

(i) said access openings being located in regions of low electric stress compared to that present at the inner periphery of said loop-shaped conductor. 

